Ahead of Tuesday's primaries, Clinton had 1,428 pledged delegates, while Sanders had 1,153. (Winning the nomination requires 2,382 delegates, including unpledged superdelegates.) In order to get on track to overtake Clinton in the delegate race, Sanders needed to win big in Tuesday's contests. That didn't happen.

Clinton easily won Pennsylvania and Maryland, aspolls had anticipated, and also took Delaware, while Sanders won Rhode Island. Clinton ﻿also eked out a win in Connecticut.

Tuesday's slate of Northeast primaries was the last big block of contests until June 7, when voters in states that include California and New Jersey head to the polls. Until then, there are a just a handful of delegate-heavy contests, including Indiana, Oregon and Kentucky.

California, with its 475 pledged delegates, could be a big opportunity for Sanders to make up some lost ground. The senator has narrowed Clinton's lead in the state, and the primary is still five weeks away. However, Sanders would still need to win by a significant margin in California and the other remaining states to pass Clinton in pledged delegates.

When you factor in superdelegates, Clinton's lead is wider: Clinton had 1,946 pledged and unpledged delegates going into Tuesday's primaries; Sanders had 1,192. While Sanders could wage a campaign to flip superdelegates to support him, it's unlikely he could pull that off if he doesn't win the popular vote.